TECHNICAL | PPC113 November 2023
Is resistance our biggest challenge in 2024? Alex Wade, from Wade Environmental, asks, “what’s stopping us?”
As 2023 seems to evaporate into a cloud, my mind turns to next year. As far as I can tell from my limited letterbox view of the industry, 2024 is going to see some interesting changes and challenges.
The withdrawal of certain products, and the loss in use patterns, continue to challenge our toolbox.
With resistance being reported across the entire UK, we have to sit up and ask ourselves: do we need to develop new strategies to manage the evolving landscape of pests and products?
So, with that in mind, the best philosophy is ‘to be forewarned is to be forearmed’. I will highlight the different resistances in rodents and insects, and give some useful practical tools to try and build strategies to combat them.
Metabolic resistance in insects
Problem
Insects are marvellous in their simultaneous simplicity and complexity. Their circulatory system is nothing more complex than a tube pumping jelly from abdomen to head and letting it wash back down the length of the body.
It’s easy to stop this process with a judicial application of insecticide, but equally easy for the insect’s own cellular defences to restart it.
In fact, the enzymes responsible for identifying and neutralising the insecticides can often be overproduced by insects.
The more they produce, the more rapidly they can neutralise the insecticide, and therefore it is more likely they will survive.
Solution
The solution to this problem is remarkably simple: metabolic synergists.
Chemicals such as PBO have no innate toxicity of their own, but they are remarkably efficient at inhibiting the enzymes which comprise the insect’s own immune system.
Products with PBO in are therefore more likely to circumvent metabolic resistance to insecticides, as well as the added benefit of making the more rapid-acting (but also generally more rapidly-flushed) insecticides more potent to even less susceptible insect pests.
Behavioural resistances in rodents
Problem
Behavioural resistance is weird. It is more of a catch-all term for when an animal acts abnormally in a way which benefits its survival in the face of adversity.
Sometimes behavioural resistance can result from an animal’s inability to process carbohydrates (and therefore to avoid carbohydrate-based baits) or it can be from learned or even shared behaviours.
In these cases, this can reward the animals with a greater chance of survival for them literally acting weird.
Solution
The solution to behavioural resistance is as simple as it is difficult. Understand what the animal does that makes it weird, and propose a strategy which does not rely on that behaviour.
Does the animal avoid carbohydrate-based baits? Use liquid baits or contact preparations, or adopt a strategy not reliant on chemicals, such as baiting traps with lures similar to the food being consumed.
If the behaviour is an avoidance borne of conditioned learning to a certain smell, location or object then the strategy needs to, where possible, eliminate those objects.
An aversion to plastics? Use wooden tamper-resistant bait boxes or cardboard where appropriate. Snap trap aversion? Consider aged wooden traps which have been on other sites or glue boards (where legal to do so).
In summary, take what makes that animal weird, and see if you can make it a solution rather than an obstacle.
Genetic resistances in insects
Problem?
Much like rodents, insects also have a penchant for mutation. In reality, the prevalence and persistence of genetic resistance in insects is much higher and can occur significantly more rapidly than we witness in rodents.
In a World Health Organisation (WHO) study, it was found that house fly populations which were treated for three consecutive visits over three consecutive generations with the same insecticide became almost functionally immune to subsequent treatments.
As insects can easily achieve multiple generations within a single season, we must take care not to proliferate these resistances.
Solution
Thankfully, despite their proclivity to foster and generate a rapid rise to resistance across generations, we have several tools which allow us to dampen the enthusiasm of our tiny crunchy adversaries.
Swapping modes of action
Much like with our genetic resistances in rodents, a swap between modes of action in insecticides is a good first step to curtail resistance.
This means that, if using a natural or synthetic pyrethroid, the logical step would be to alternate to a carbamate or organophosphate - although the number and availability of such products is becoming vanishingly low (perhaps a conversation for another time).
The use of insect growth regulators
IGRs are slow burners. They have little effect on an insect population in terms of true mortality. They will not kill, but simply reduce the ability to grow, pupate or reproduce.
This denies the insects the ability to give subsequent generations the genetic mutations to resist your chemical treatments.
Heat
Finally, with this (and in fact all insect resistances) the move away from chemicals to a physical mode of action will bypass any resistance that the insect has.
Steamers, whole room and chamber heat treatments will therefore be tremendously effective against stubborn populations of insects.
Genetic resistances to rodenticides
Problem
We are seeing increasingly widespread resistance to the multifeed second generation anticoagulant rodenticides (SGARs) in numerous locations across the UK.
These resistances have formed through the inheritance of mutant genes which confer varying degrees of tolerance to the chemicals we use.
Some resistant genes are relatively weak and only cause a minor loss in efficacy to one or the other of the anticoagulant rodenticides.
Others (such as the fabled L120Q) will bequeath a more robust tolerance, making the field application of both difenacoum and bromadiolone technically problematic.
Solutions
You will find that with many of the solutions discussed, knowledge is power. This is especially true when it comes to circumventing genetic resistance in rodents.
The first step is to know precisely what it is that you are dealing with. This can often be easily (and quickly) achieved by checking the Rodenticide Resistance Action Committee (RRAC) website/app and ascertaining the situation in your local area.
From there, the most expedient routes to success are:
Alternate between multifeeds
With all the resistances documented in rodents to date, only the L120Q variant shows resistance to both of the multifeeds. Therefore, if you are having control issues with difenacoum, try swapping to bromadiolone or vice versa.
Escalate in potency
If alternating between multifeeds doesn’t seem to work despite good engagement with bait, then consider escalating from a multifeed to a single feed SGAR.
Although increases in tolerances are noted to some single feeds in the more potent strains of resistance, not one strain shows there to be a functional field resistance and so the strategy of escalation holds firm.
However, we must remain mindful: a reliance on any one chemical is likely to generate resistance over time.
Swapping between modes of action
If neither strategy A nor B seem to be effective or appropriate for the area of the rodent control programme, then swap to a product with an alternative mode of action.
Either cholecalciferol or alpha-chloralose are able to circumvent most resistances borne of the hereditary mutations in rodents, as often these genetic resistances effect only isolated sections of the animal’s biology.
This means that something that gave resistance to SGARs is unlikely to provide resistance to an overdose of Vitamin D nor the narcotic effects of alpha-chloralose.
Last word on resistance in 2024
The major thing we need to keep in mind to control any resistance we encounter is simply this: understand what gives the pest an advantage and remove it! It’s as simple and, as potentially brutally complicated as that.